An image diagnosis which is based on images conveying the information of a lesion is an indispensable method for clinical diagnosis. In addition to an X-ray CT which is one of the image diagnoses currently in wide use, new distinguished techniques for image diagnosis such as Magnetic Resonance Imaging (MRI) have been developed for the last decade or so, and are making great contribution to the development of image diagnosis.
MRI has been recently introduced into the medical field and rapidly improved to be widely used ever since. MRI is advantageously characterized in that it is free of exposure since it does not involve radiation, that optional cross section can be put into an image and that it is free of hindrance by bones, and these characteristics make MRI distinct from conventional X-ray CT. MRI shows magnetic resonance phenomena [usually a relaxation time (T.sub.1, T.sub.2) of hydrogen atomic nucleus] of internal substances with different signal intensities. A paramagnetic substance promotes relaxation of proton (proton of water) and acts as a contrast medium capable of enhancing contrast of images. In particular, rare earth 6d (trivalent) has 7 unpaired electrons on the 4f orbit and has many coordinations (9 or 10), which results in a strong relaxation effect to provide a powerful contrast medium [R. B. Lauffer, Chem. Rev., 87, 901 (1987)]. However, 6d (trivalent) is not discharged from the body and poses toxicity problem.
For this reason, Gd is administered as a complex compound (Gd-DTPA) with a known chelating agent DTPA (diethylenetriaminepentaacetic acid).
Gd-DTPA has been acknowledged to be useful for clinical diagnosis. However, there are many problems to be resolved with respect to Gd-DTPA. For example, the drug per se has a short half-life in blood and poor tissue selectivity and shows a high osmotic pressure since it is present as a bivalent anion complex under physiological conditions. While various approaches have been taken to overcome these problems (Japanese Patent Unexamined Publication Nos. 93758/1988, 1395/1989), they have not necessarily achieved satisfactory results.
Accordingly, research and development of new complex compounds, particularly of a chelating agent, is significantly important.